There are many thermoplastic polymers which can be made into fibers and filaments. Of particular interest are polyolefins which include polyethylene, polypropylene, polybutene, polypentene, and ethylene and propylene copolymerized with other olefinic monomers such as higher olefins and conjugated dienes. Olefin polymers are known for their hydrophobic properties. Therefore, wettability of such polymers, including those in fibrous or filament form, is achieved by means of wetting agents provided in, or on, the polymer fibers or filaments.
Such fibers or filaments are useful in producing battery separators (e.g., U.S. Pat. No. 3,847,676); disposable absorbent products such as diapers, feminine care products, incontinence products and the like (e.g. U.S. Pat. No. 4,073,852 and U.S. Pat. No. 4,923,914); wiper materials (e.g., U.S. Pat. No. 4,307,143); papers (e.g. U.S. Pat. No. 4,273,892); and filter aids (e.g., U.S. Pat. No. 4,274,971).
Synthetic fibers are produced as continuous filaments by a process known as melt spinning. Plastic pellets are fed in to a hopper and melted in a single screw extruder. The molten polymer is then filtered and metered before being forced through a spinerette that contains thousands of small holes to form fibers. The fibers are then solidified by coming into contact with air as they are drawn by the rollers of sequentially increasing speeds. The drawn fibers are then wound onto spools. The spools are made into bunches called tows and cut into staple fibers according to the manufacturer's requirements.
Fibers are the main constituents of fabrics and nonwovens. Nonwovens are unconventional textile assemblies that are obtained by processes other than weaving. In recent years, there has been is a phenomenal growth in the usage of nonwovens in personal hygiene, diapers, adult incontinence, medical, construction, geotextiles and automotive applications. Nonwovens are made from both natural and synthetic fibers and a combination of both. Usually, the fibers are obtained or extruded and bonded into thin sheets by heat or mechanical or chemical means. The main types of nonwovens used in the market includes spunbonded and melt blown type. The details of these types of materials and their manufacture will be explained in the following paragraphs. Very often, more than one type of nonwoven is used to make laminates or composite structures. The nonwovens are sometimes further bonded to provide them with adequate strength for other operations.
In the manufacture of a spunbond nonwoven, polymer chips are fed through the hopper and melted in the single screw extruder. Some machines also have side feeder capability to feed various additives. The molten polymer is metered and then forced into fine continuous filaments through several thousand spinnerette holes to form continuous filaments. The filaments are drawn and entangled through the action of venturi and deposited on a collection belt. The unbonded, entangled fibers are passed through two heated calendar rolls for thermal bonding of the fibers to each other at points of contact. The nonwoven fabric is then wound and shipped to converting applications where the fabric is made into final products. Alternatively or in addition, the fibers may be bonded through needle-punching and chemical bonding. The polymers typically used in spunbonding are polyolefins, such as polyethylene and polypropylene; polyamides; and polyesters. Polypropylene is most commonly used with a melt flow rate in the range between 30-40. Polypropylene is easy to process and cost-effective when compared to other polymers. Spunbond polypropylene nonwovens are used in baby diapers, napkins, feminine hygiene products, laminates, adult incontinence, medical garments, agricultural covers, etc.
In the manufacture of a melt blown nonwoven, polymer chips are fed through a hopper and melted in a single screw extruder. Some machines have additional side feeding capacity for additives. The molten polymer is forced through very fine die holes that are situated either vertically or horizontally to form fibers. The fibers are then subjected to the action of very hot air at very high velocities which results in fibers with sub-micron diameters. The fibers are bonded to each other at contact points as they cool down. Usually, there is no separate bonding process. Melt blown products are used in filters, wipes, battery separators, and insulators.
The use of surfactants in textile fibers is well known. For instance, the surfactants have been used as spin finish to provide cohesion between synthetic fibers before drawing and texturing. This also reduces the friction between the rollers and fibers preventing abrasion of rollers and breakage of the fibers. Surfactants are also added to the finished fiber goods to impart the desired finish, for example, hydrophilic, hydrophobic, or oil repellant. The surfactants could be cationic (quaternary ammonium compounds), anionic (phosphates, sulfates) or non-ionic (esters, alcohols, ethoxylates, etc.).
However, their use has been mainly through topical surface coating treatments such as spraying, coating, padding, etc. A similar approach has been used conventionally in woven or knitted fabrics and nonwovens. Surface coating applications have a number of disadvantages including the following: 1) throughput is reduced and more floor space is required; 2) when spraying is involved, the over spray and the spills are environmental concerns; 3) the coating is not usually well-bonded to the fiber and may be partially lost during storage or in subsequent operations; and 4) there are always some quality control issues, for example, the uniformity of the coating. Examples of such methods and products are discussed below.
U.S. Pat. No. 3,929,509 (according to U.S. Pat. No. 4,923,914) discloses a hydrophilic microporous film which is useful as a battery separator. The film comprises a hydrophobic microporous film coated with a silicone glycol copolymer surfactant. In a preferred embodiment, the surfactant coating comprises a mixture of a silicone glycol copolymer surfactant and a second surfactant which preferably is an imidazoline tertiary amine. The silicone glycol copolymer surfactant preferably is a polyoxyethylene polydimethylsiloxane.
U.S. Pat. No. 4,307,143 discloses, inter alia, a wipe made of meltblown microfiber webs which have been sprayed with a wetting agent as the web was formed. The fibers exemplified were of polypropylene and of polyethylene terephthalate polyester. The wetting agents disclosed were dioctylester of sodium sulfosuccinic acid (exemplified) and isooctyl phenylpolyethoxy ethanol (not exemplified).
Recent advances in this area have been to incorporate the additives in the melt state in attempts to form a melt-stable formulation. One such additive is the use of melt blendable surfactants in forming hydrophilic nonwovens or fibers. The melting point or molecular weight of the additive determines its processability during its incorporation into polymers and the processability at the spinning or blowing stage. The lower the molecular weight, the lower the viscosity and this governs the amount of liquid that can be incorporated. Two things to keep in mind while melt blending and using the melt blended concentrate for final goods manufacturing are incorporation and migration characteristics: 1) incorporation at the melt state depends on the solubility of the additive with melt, additive- and polymer-type, chemistry, polarity, molecular weight, melting point, etc.; and 2) migration of the additives to the surface depends on the diffusion characteristics of the additive in the solid state, molecular weight, structure, purity, etc.
As long as the above mentioned parameters are carefully selected for a particular application, one will be able to achieve the right type of surface modification and additive incorporation. However, this is typically easier said than done. Substantial effort has been expended to achieve such properties and improvements as evidenced by the patents in this area. Along this line, various types of additives and their mixing with different types of polymers in the melt stage have been provided in the previous arts.
U.S. Pat. No. 3,048,266 discloses, inter alia, a polyolefin film containing an anti-fog agent, which is a particular ester or ether of ethylene oxide. The anti-fog additive is preferably incorporated in the polyolefin material rather than sprayed on.
U.S. Pat. No. 4,578,414 discloses an olefin polymer, preferably a linear low density polyethylene copolymer (LLDPE), having compounded therewith a wetting agent, which is used in forming wettable fibers and/or filaments. The wetting agent has at least one of the following: (1) an alkoxylated alkyl phenol along with a mixed mono-, di- and/or tri-glyceride, or (2) a polyoxyalkylene fatty acid ester, or (3) a combination of (2) and any part of (1).
U.S. Pat. No. 4,923,914 discloses, inter alia, a thermoplastic composition containing a thermoplastic polyolefin and an additive which is a siloxane-containing compound. During the processing of the composition as the thermoplastic polyolefin cools down, there is a segregation of the additive towards, for instance, the surface of the fiber as the polymer gradually solidifies. One such compound is identified therein as being commercially available as SILWET L-7602 from Union Carbide. (See Column 22, lines 62-63 thereof). These compositions may be used to form non-wovens according to the methods of U.S. Pat. No. 4,857,251, which also identifies SILWET L-7602 (See Column 23, lines 59-60).
However, to date, the efficacy of these surfactants is generally limited by their poor wettability and durability. Generally, wetting agents having high wettability are readily washed from the fiber exactly because of the wetting agent's hydrophilic characteristics.
An attempt to provide hydrophilic polypropylene fibers having an improved balance of wettability and durability is disclosed in U.S. Pat. No. 3,847,676 relating to battery separators. This patent discloses the use of an internal melt blendable surfactant having moderate wetting action so that it will not easily be removed, thus having higher wetting action producing durable, hydrophilic fibers. Particularly preferred internal surfactants were C.sub.8 and C.sub.18 phenol surfactants having 1 to 15 moles of ethylene oxide, more preferably 1 to 6 moles of ethylene oxide and most preferably 1 to 3 moles of ethylene oxide. These surfactants are relatively water insoluble. However, because of the limited wetting action of the internal surfactant, a second surfactant used as an external surfactant was coated on the surface of the fibers in order to enhance the hydrophilic performance of the fibers. The external surfactants are relatively water soluble. In Example 1 thereof, the internal surfactant was nonyl phenol ethylene oxide containing 4 moles of ethylene oxide; and the external surfactant was a mixture in equal parts of Triton X-100 (an isooctyl phenyl polyethoxy ethanol per U.S. Pat. No. 4,307,143) and dioctyl ester of sodium sulfosuccinic acid. This two-step method is generally complex and expensive, limited by the same problems associated with the surface coating of wetting agents and which were described earlier.
Further, a fine point that is missing is the identification of cost-effective formulation of the concentrates or master batches and the way to commercially produce them. As mentioned previously, there is a critical limit on the amount of additive that can be incorporated which depends on the type of additive, type of polymer and the type of equipment employed to produce a commercially viable concentrate that can cater to the commodity goods market.
Along this line, surfactants have also been blended with the plastic at the melt stage at higher concentrations than that in the final article. This material is then cooled and sold in the form of pellets known as additive concentrates or master batches. The use of a master batch in fiber spinning, nonwoven manufacturing and other related processes to modify the surface is dependent on the final application, migration of the material to the surface, polymer and additive characteristics, process environment and cost. The nonwovens industry that supplies the diaper and hygiene markets has always been on the look-out for a cost-effective additive formulation that is durable and with high re-wet characteristics.
More often, depending on the application, the manufacturers of thermoplastic fibers and nonwoven manufacturers are confronted with the production of hydrophilic materials that should have a high degree of durability during storage, washing, dyeing, finishing and in end use applications. Natural fibers such as cotton and viscose rayon have excellent water absorbing capabilities but retain moisture for a long period of time. Cotton fibers can pickup as much as 8% by weight water and viscose rayon twice that amount. In cotton, the exceptional water-uptake can be attributed to the presence of three hydroxyl (--OH) groups. Also, the amorphous regions in the fibers are believed to be primarily responsible for water or dye-uptake. The fabrics are usually not compact and the fibers wither away during usage, especially in hostile environments such as acids or alkalis. The invention of synthetic fibers such as polyamide (nylon) and polyester in the 1960's has resulted in the production of fibers where required properties could be tailor-made. These fibers can be considered as moderately hydrophilic. Nylon has a water regain of about 4% by weight and polyester of about 0.4% by weight. Moreover, the presence of functional groups, i.e., --C(O)--N(H)-- in nylon and --C(O)--O-- in polyester can be exploited in dyeing with acidic or basic dyes. The same groups are responsible for their water absorption characteristics. These fibers have better environmental resistance when compared to the natural fibers. However, they also degrade over a period of time when exposed to acidic or alkaline environments.
The solution for this problem has been to use fibers made from addition polymers such as polyolefins. Among the polyolefin polymers, polypropylene is the most widely used because it is cheap and has ideal rheological characteristics essential for fiber formation with high crystallinity. Thus, fibers made from polypropylene polymer have been replacing natural and other synthetic fibers for economical, property and processing reasons. However, in situations where hydrophilicity is needed, the surface of polyolefin polymers must be modified or altered through the use of wetting agents.
Finishing operations during textile processing permits the surface and even some times the bulk of the material to be modified to make soft, hydrophilic, hydrophobic and oil repellent goods. However, the surfactants used are usually only mechanically applied and hence get washed or wiped away during further processing, usage or storage. This effect is especially severe in the case of a non-polar, hydrophobic polymers such as polyolefins where the interaction between the surfactants and the polymer is minimal. The surfactant is usually a liquid, gel or a solid that is usually formed into a solution that is then applied onto the polymer. Water or alcohol can be used to form the solution. The solution is applied through spraying or padding, and is then dried. This is a labor and energy intensive process which results in significant loss of surfactants and potential environmental problems. More recently, the liquid or solid additives were melt blended with plastics that can be directly extruded into films, filaments, fabrics or nonwovens. The surfactants are usually loaded at a much higher concentration in the plastic using a special process where sometimes, the liquid is injected into molten polymer and extruded. This concentrate is used at a certain percent by weight by the end use manufacturers to achieve the desired properties.
The use of melt blendable additives is to delay the loss of the surface active agents. The polymer acts as a reservoir for the additives that get pushed to the surface of the final part during the slow recrystallization process. Theoretically, the use of surfactants that are in liquid form in the melt form a separate phase within and on the surface of the polymer. This and the incompatibility of the part of the surfactants pushes the low molecular weight additives to the surface. This phenomenon is dependent on the type of alkyl chain that is miscible with the plastic, the molecular weight of the additive that determines processability, stability and migration and the type of polymer itself.
Therefore, a need exists for wetting agents which resist migration and transference, but provide wettable fibers or filaments which do not require aging, and are easily incorporated and commercially viable. In other words, to date, a need remains for more durable, hydrophilic compositions for fibers which are easy and economic to use.